Because we never will know everything, we all need to believe.
Free world people know that we all need believing. And that being conscious of this fact is an essential ingredient for building a peaceful society.
Stories in books synchronize believes but never can be "the truth" or "the law" because if words would be the truth, we all would speak the same language.
We believe for we can not know everything. Some of us may need to personalize our believe in gods or one god. We "Free world people" give every individual the freedom to believe what they need and expect every body to respect all forms of believe.
Believing is the opposite of knowledge. Also when it is written in books. We can tell each other stories to synchronize our believe systems, but we have to respect any individual deviation from what we think as being common believe, because believing is individual. It is a form of the highest respect for the individual to accept that we do not all know the same things, and therefore can not have the same believes. This respect for each individual beliefs is Free Humanism. And it is personal and thus never can be in books.